问题 多选题

质量为m的小球由轻绳a和b分别系于一轻质木架上的A点和C点,如图所示,当轻杆绕轴BC以角速度ω匀速转动时,小球在水平面内做匀速圆周运动,绳a在竖直方向,绳b在水平方向,当小球运动到图示位置时,绳b被烧断的同时杆子停止转动,则(  )

A.小球仍在水平面内做匀速圆周运动

B.在绳b被烧断瞬间,a绳中张力突然增大

C.若角速度ω较小,小球在垂直于平面ABC的竖直平面内摆动

D.若角速度ω较大,小球可能在垂直于平面ABC的竖直平面内做圆周运动

答案

A、小球原来在水平面内做匀速圆周运动,绳b被烧断后,小球在垂直于平面ABC的竖直平面内摆动或圆周运动.故A错误.

    B、绳b被烧断前,小球在竖直方向没有位移,加速度为零,a绳中张力等于重力,在绳b被烧断瞬间,a绳中张力与重力的合力提供小球的向心力,而向心力竖直向上,绳b的张力将大于重力,即张力突然增大.故B正确.

    C、若角速度ω较小,小球原来的速度较小,小球在垂直于平面ABC的竖直平面内摆动.故C正确.

    D、若角速度ω较大,小球原来的速度较大,小球可能在垂直于平面ABC的竖直平面内做圆周运动.故D正确.

故选BCD

阅读理解

阅读理解。

     Colors can be seen everywhere. We see them in the sky and in the sea. There are colors in rocks and

in all plants and animals. Everything we see has colors. We see the colorful world. How can we see colors?

We see colors when light hits our eyes. Dogs and many other animals can't see colors. But bees can. This

helps them find flowers to fly to.

     We sometimes use the means of colors to express our feelings. People may say that they see red when

they are angry. Or you may say you feel blue. This means you are sad. We also use colors to tell us things.

Red is the color of stopping signs. Green means that you can go.

     Animals come in all kinds of colors. Many of these are very pretty. An animal's color may say "Here I

am." or "Watch out". Or it may help the animal to hide. Some animals can make their colors similar to their

surroundings (周围的事物). Then they can't be seen easily.

     Some people can't tell one color from another. They can see yellow and blue. But they can't tell red from

green. We say these people are color blind.

1. We can see colors _____.

A. in the sky

B. in the sea and rocks

C. in all plants and animals

D. A, B and C

2. If your friend is sad, he may say "_____".

A. I see red

B. Watch out

C. I feel blue

D. I feel yellow

3. According to the passage, why sometimes some animals can't be seen easily?

A. Because they have special colors.

B. Because they make their colors different from other animals.

C. Because they have bright colors.

D. Because they make their colors look like things around them.

4. The person who _____ is color blind.

A. can see yellow and blue

B. can't tell red from green

C. can't see yellow and blue

D. can tell red from green

5. What's the best title of the passage?

A. Colors

B. Color Blind

C. We See Colors

D. Animals and Colors

填空题